Current state-of-the-art commercial airborne weather radar systems typically provide display of the following information: Weather Reflectivity, Ground Reflectivity, and optionally, Turbulence. The displayed information is determined from radar returns (radar signal reflections from incident radar signals emitted by the aircraft weather radar system).
Weather reflectivity is essentially a measure of the “percentage” of a reflected radar signal returned back to the radar receiver onboard the aircraft. Weather reflectivity information provides an approximate indication of rainfall rate when the radar signal is incident on weather in proximity to the aircraft. The weather reflectivity information is usually graphically displayed on a display of the radar system as areas of green, areas of yellow, and/or areas of red, where the green, yellow and red colors correspond to increasing levels of reflectivity. For instance, the crew of an installation aircraft may understand that an area shown as a green colored area of reflectivity indicates a light level of precipitation (that is presumably safe to fly through). The crew understands that another area shown as a yellow colored area of reflectivity indicates a higher level of precipitation that, although safe to fly through, will have noticeable levels of precipitation. And, the crew understands that an area shown as red colored area of reflectivity indicates a high level of precipitation (that the aircraft should be flown around if possible). Other colors may be used, such as magenta or the like, to indicate levels of severity with higher granularity.
Ground reflectivity may also be a measure of the percentage of a returned radar signal of the ground. However, the particular value of the ground reflectivity information does not correspond to particular aviation-operationally relevant characteristics, other than, for example, to provide a distinction between land masses and large bodies of water.
Broadly defined, turbulence is random or chaotic motion. In the context of aviation, turbulence generally relates to random motion of air. More precisely, the presence of turbulence means that the velocity of air in a region of interest is not correlated spatially. That is, non-turbulent (smooth or laminar flow) air has the characteristic that the velocity vector (magnitude and direction) of the air, for the most part, is the same over a large volume. In contrast, turbulent air is characterized by a high degree of variation in wind speed over relatively short length scales (i.e., the velocity vectors in the wind field at two points of a relatively small displacement are not correlated). If turbulence information is presented on the display, such information is typically inferred from various characteristics of the returned radar reflections and/or may be based on Doppler information contained in the return signals.
FIG. 1 is a conceptual illustration of a portion of a weather radar display 102 showing hypothetical weather reflectivity information 104 obtained by a conventional weather radar system onboard an aircraft. Here, two regions of weather, or weather cells, are indicated on the portions of the weather radar display 102. A first weather cell graphic element 106 is shown on the display 102 using a green color (conceptually denoted with the “green” text shown thereon) to indicate the location of a first weather cell. A second weather cell graphic element 108 is shown on the display 102 using a red or yellow color (conceptually denoted with the “yellow/red” text shown thereon) to indicate the location of a second weather cell.
The first weather cell graphic element 106, for discussion purposes, is understood by the crew to be associated with stratiform weather. Stratiform weather is characterized by relatively low amounts of water volume, low vertical air velocities, and/or relatively low circulation confined to a small band of altitudes. Often, stratiform weather is generated by a slow, steady air mass lifting process, either orographically or by one air mass lifting another. Stratiform weather can also result as outflow from convective weather. The radar signal reflectivity level of stratiform weather is relatively low. This low level of radar signal reflectivity may be indicated using a green color in the hypothetical example illustrated in FIG. 1.
The second weather cell graphic element 108, for discussion purposes, is understood by the crew to be associated with convective weather. Convective weather is characterized by significant lifting and circulation of air (i.e., significant updrafts and down drafts) existing over a relatively large band of altitudes. Convective weather may be associated with several hazards including turbulence, lightning, and/or hail. Also, convective weather is typically associated with a relatively high water content. Accordingly, the radar signal reflectivity level from convective weather is relatively high. This higher level of radar signal reflectivity may be indicated using a yellow and/or red color in the hypothetical example illustrated in FIG. 1.